Geology Reference
In-Depth Information
dc
dt
K
:
It may be noted that the process is almost linear up to 80 % conversion, then the
activation energy is in the order of 8 kcal mol 1 and certainly less than 10 kcal. This
is very low activation energy and the reaction will be rather temperature-insensitive.
If we consider that the time constants of diagenetic (burial) process is in the order of
10 5
10 7 years then even at 25
°
C given appropriate
fluids, the reaction would go to
-
completion.
The process studied was:
Na þ aq
K þ aq
KAlSi 2 O 6 solid
ð
Þþ
þ
H 2 O
,
NaAlSi 2 O 6 :
H 2 O solid
ð
Þþ
G 298 ¼
1093 mol 1 Robie and Waldbaum
ð
1968
Þ :
D
;
G o , this
figure suggests that analcite
is stable even when the concentration of potassium exceeds the concentration of
sodium (K a + /Na a + = 6.4). This implies that even normal river water (K + /Na + = 0.3)
could cause the reaction.
Although no nucleation is involved in the reaction (but a 10 % volume
expansion occurs) a number of steps could be rate-determining which might
include:
1. diffusion rates of Na + or K + in the crystal,
2. dehydration rates of ions before entry, and
3. diffusion rate of water into the crystal.
While there is considerable uncertainty in
Δ
The overall H (cf. E) of processes like:
K solid þ
Na aq !
Na solid þ
K aq are small Larimer 1964
ð
Þ :
Normal heats of hydration of solids are similar to the activation energy deter-
mination in their work. The fact that more dilute seawater appears to react faster
than the saturated salt solution, it may indicate that the water activity in diffusion is
important.
Other authors also noted the facile leucite-analcite reaction (Deer et al. 1963;
Bragg et al. 1965). The chances for leucite survival over a long geological period
are therefore, small and in fact most reported occurrence of volcanic leucite are
rather in recent rocks. It is therefore very much possible that this process may occur
at low temperatures when other primary igneous materials might show little
alteration. This possible reaction must therefore be considered before analcite is
thought to be a primary igneous phase (Pearce 1970). Nakamura and Yoder (1974),
who studied analcite
in basalts, also concluded that they are most
likely exchange products of original leucite.
Macdonald et al. (1992) studied turbid, brown rounded pseudoleucite pheno-
crysts from Bearpaw Mountains, Montana. They found that
phenocrysts
the pseudoleucite
 
Search WWH ::




Custom Search